PSI - Issue 62

Michele Palermo et al. / Procedia Structural Integrity 62 (2024) 593–600 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2. Sills and gabions The presence of a sill or a gabion contributes to significantly modify the sour features and their evolution kinetics, especially in the presence of large wood debris accumulations. Some of the key studies on the topic were conducted by Grimaldi et al. (2009a) and (2009b). Namely, Grimaldi et al. (2009a) conducted tests in two different channels under incipient motion conditions, i.e., for U/U c =1, with U indicating the average velocity and U c the threshold velocity. The bed sill was located at L/D ranging from 0 to 6.7, where L denotes the distance of the sill from the pier and D the diameter of the pier. The authors found that the effectiveness of the sill in reducing the maximum scour depth z max is significant (up to 26%) but decreases with L. Furthermore, the presence of the sill significantly contributes to vary scour geometry, leading to a maximum reduction of scour area and volume larger than 80%. Finally, they evidenced that the effect of the sill can be detected after a certain time from the scour beginning in the range 0 ≤ L/D ≤ 2. Successively, Grimaldi et al. (2009b) tested the effect of a slotted pier and sill on the scour process, revealing that the combination of the two countermeasures is effective in reducing z max , i.e., the average reduction was equal to 45%. Nonetheless, the mentioned studies did not account for the presence of debris accumulation. Such configuration was deepened by Pagliara et al. (2010a). The authors conducted tests for U/U c =1 and 0 ≤ L/D ≤ 0.5 in the presence of two different types of debris, two sills (S1 and S2) and one gabion (G1). Figures 1a and b show diagram sketches of the experimental apparatus adopted by Pagliara et al. (2010a), along with the main hydraulic and geometric parameters, with b indicating the channel width, and l d , t d and d d are the length, the thickness, and the width of the debris. (Note that the adopted debris were characterized by l d /D equal to 5 and 3; therefore, in the following, we will term them as debris 5D and 3D, respectively). Figure 1c shows a picture of the employed sills and gabion, whereas Figures 1d reports a picture of a test with gabion G1.

Fig. 1. (a) Side and (b) frontal views of the experimental apparatus, along with the indication of the main geometric and scour characteristics; (c) picture of the sills/gabion employed; (d) particular of the scour formation for a test. All the figures refer to tests conducted by Pagliara et al. (2010a).